Thus the Exiles of Númenor established their realms in Arnor and in Gondor; but ere many years had passed it became manifest that their enemy, Sauron, had also returned.

The Silmarillion, Of the Rings of Power and the Third Age

The last leaders of the Faithful, Elendil and his sons, escaped from the DownFall with nine ships, bearing a seedling of Nimloth, and the Seven Seeing-stones...; and they were borne on the wings of a great storm and cast up on the shores of Middle-earth. There they established in the North-west the Númenórean realms in exile, Arnor and Gondor. Elendil was the High King and dwelt in the North at Annúminas; and the rule in the South was committed to his sons, Isildur and Anárion. They founded there Osgiliath, between Minas Ithil and Minas Anor, not far from the confines of Mordor. For this good at least they believed had come out of ruin, that Sauron also had perished.

But it was not so. Sauron was indeed caught in the wreck of Númenor...; but he fled back to Middle-earth.... [His] anger was great when he learned that Elendil whom be most hated, had escaped him, and was now ordering a realm upon his borders.

The Lord of the Rings, Appendix A, Annals of the Kings and Rulers: Númenor